Waveguide hybrid junctions



t- 0, 1970 E. mam 3,535,559

WAVEGUIDE HYBRID JUNGTIONS Filed Hatch 11, 1958 United States Patent O 3,535,659 WAVEGUIDE HYBRID JUNCTIONS Edward Salzberg, 19 Black Oak Road, Wayland, Mass. 01778 Filed Mar. 11, 1968, Ser. No. 711,947 Int. Cl. HOlp 5/12 US. Cl. 33311 4 Claims ABSTRACT OF THE DISCLOSURE Direction hybrid couplers in rectangular waveguide are described. Slot coupling is used in a common narrow wall with a longitudinal slot narrower than the height of the narrow wall and at least as high at the center as at the ends.

BACKGROUND OF THE INVENTION One of the best known prior art hybrid junctions is that described in US. Pat. No, 2,739,287 issued to Henry J. Riblet. As defined in that patent, a waveguide hybrid is a waveguide circuit having four (two input and two output) Waveguide terminals, which has the property that energy incident on one of the input terminals will divide evenly between the two output terminals with only a small fraction of the energy escaping through the other input terminal.

In general, the less energy escaping out of the other input terminal, that is the greater the isolation between the two input terminals, assuming matched terminations on the output terminals, the better the hybrid. Also, the evener the power division at the output terminals, the bet ter the hybrid. Other criteria of merit are the bandwidth over which the hybrid will operate and the peak power handling capability.

SUMMARY OF THE INVENTION Riblets coupler uses a coupling aperture in the common narrow wall that is substantially equal to the height of the narrow wall and has centrally disposed capacitive loading means which effectively narrows the slot at its center. In accordance with the present invention, it has been found that using a simple aperture, a relation between the aperture height and the height of the narrow wall can be found that yields a broader band response without the addition of centrally disposed capacitive loading means. Using the inventive technique, it has also been found possible to operate a hybrid of given waveguide dimensions in a lower frequency portion of the waveguide band than previously accepted.

Thus it is an object of the invention to define a novel directional hybrid waveguide coupler.

It is a further object of the invention to define a novel hybrid waveguide coupler which has a longitudinal coupling aperture in a common narrow wall with a height less than the height of said narrow wall and at least as high at its center as at its ends.

Further objects and features of the present invention will become apparent on reading the following specifications together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of one embodiment of a hybrid junction according to the invention partially cut away to show the coupling aperture.

3,535,659 Patented Oct. 26, 1970 "ice FIGS. 2-6 are perspective views of alternate common wall configurations for the junction of FIG. 1.

FIG. 7 is a perspective view partially cut away of a second embodiment of a hybrid junction according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The hybrid or directional coupling, as depicted in FIG. 1, has two input terminals 10 and 11 as well as two output terminals 12 and 13. When energy within the bandwidth of the junction is applied at one input terminal, for example terminal 10, it ideally divides between output terminals 12 and 13 with no output at input terminal 11 or reflection at input terminal 10.

The hybrid junction of FIG. 1 is made of two sections of rectangular waveguide joined by common narrow wall 15 so that the two sections of waveguide are parallel to each other. In this configuration, the two waveguide sections form a single substantially rectangular double-width section. Common wall 15 includes means for coupling energy between the two sections. In the present invention, this coupling means is a slot aperture in the common narrow wall depicted in FIG. 1 as aperture 16. Since this aperture leaves ridges of common wall 15 bounding it top and bottom, it is designated herein as a slot-ridge aperture.

The length of the junction and the position of aperture 16 longitudinally in wall 15 is a question of design determined by the intended utilization. For design ease, a structure of this general type is often, but not always, made symmetrical about the common wall.

Aperture 16 has its greater dimension longitudinally in wall 15 and a height that is less than the height of wall 15. Aperture 16 depicted in FIG. 1 is rectangular. Sometimes it has been found desirable to round the corners slightly and, as depicted in FIG. 2, common wall 15 may have a more nearly elliptical aperture 17.

The dimensions of the slot-ridge aperture are critical in' the present invention, however the height and length of the aperture interact such that with greater height the length should be greater and vice versa. The length/ height ratio ultimately determines the bandwidth according to the invention so that this ratio is tailored for the desired bandwidth within limits as will be described.

Generally the aperture is determined in accordance with the operating wavelength. The length of the aperture should approach the free space wavelength at the design midband frequency and should be in the range of to of such wavelength. Within this range, the slots should be longer with thicker septums (common walls). The height of the aperture must be selected to balance the power between the output terminals.

Following are two examples of apertures for junctions operating in the range of 13.5 to 15.5 gigacycles.

EXAMPLE I A junction as depicted in FIG. 1 was made of adjoining waveguide sections each .510 inch wide by .255 inch high (inside dimensions). The slot-ridge aperture was made .760 inch long by 142 inch high and positioned centrally in the common narrow wall. The corners of the aperture were rounded with a .07 inch radius. The common narrow wall thickness was .040 inch. The test setup was as follows:

A signal generator was connected to input terminal 10 through a variable attenuator.

A matching pad and a bolometer were connected in turn to each of the other three terminals with the unused terminals terminated with refiectionless terminators. Balance is given as the output at terminal 13 relative to terminal I2. Directivity is given as the output at terminal 13 relative to terminal 11.

The following readings were taken:

Frequency, g.c.s. Balance, db Dircctivity, db

*The test; setup provided a maximum calibration of 40 db and the asterisk indicates a substantially higher reading.

EXAMPLE II Using the same setup as in Example I, the aperture was increased in length to .770 inch. The following readings were taken:

Frequency, g.c.s. Balance, db Directivity, (lb

Of interest in these two examples are the double peaks of directivity with a shallow dip in between. In Example I, these peaks are at 14.0 and 15.0 gc.s. In Example II the peaks are at 13.5 and 15.0 g.c.s. showing a greater spread when the aperture is elongated slightly. It is also noteworthy that the direction of spread is almost entirely toward lower frequency.

The theory of operation generally is believed to be that coupling, in a hybrid junction of the general type described, takes place due to the existence of two modes known as the odd and even modes. Optimum directivity is obtained when both modes are perfectly matched. Power in the two modes cancels toward the inputs and divides toward the outputs.

Even mode matching is the most difiicult because the even mode has a voltage peak at the coupling aperture requiring a critical impedance match.

In prior techniques, as defined by the aforementioned Riblet patent, the inductance of the aperture ends was compensated for by introducing a centrally located capacitance. This apparently produced a perfect match at one frequency only. Going directly against the Riblet teaching by decreasing the height of the aperture ends and leaving longitudinal ridges above and below the aperture for the full length of the aperture, the coupler of the present invention matches the even mode at two frequencies.

By careful contouring of the ridge around the slot, it is possible to flatten the response curve of the inventive junction so as to virtually eliminate the dip between the two peaks. This contouring is best done empirically due to the large number of variables involved. A first approximation is obtained by proportionate scaling, for instance from one of the examples given herein. If improvement is desired, the ridges can be machined back in small amounts until optimum performance is obtained. (It wi l be understood that to permit this it is preferable to make the aperture somewhat smaller than exact scaling for the first approximation.) Once optimum performance is achieved, the prototype can be used to make casting dies for quantity production.

With the ridge-slot technique, good hybrid operation has been obtained over bandwidths greater than fifteen percent of the center frequency and with a lower low frequency limit than previously achieved for a given size of waveguide.

FIGS. 2 through 6 show variations of the ridge-slot aperture the common wall 15. FIG. 2 shows elliptical slot 17 (concave ridges) while FIG. 3 shows slot 23 with fiat ridges and well radiused corners. In practice, variations from the rectangular are slight and these figures serve to show the contouring that can be introduced to flatten the response curve. Optimum contouring can be readily found experimentally and will depend on such things as the thickness of common wall and the aspect ratio of the waveguide sections.

FIG. 4 depicts two slots 24 and 25 with ridges 26 and 28 on the top and bottom and a separating strip 27 in between. The number of slots is not critical. When multiple slots are used, the ratio of total aperture height (sum of the individual aperture heights) to the narrow wall height should still be approximately the same as with a single aperture.

FIGS. 5 and 6 depict a slot-ridge configuration for greater power. Sharp protrusions are the first to provide a voltage breakdown. Thus slot 27 of FIG. 5 is bounded by ridges 28 and 30 that are rounded and extended onto the broad walls of the waveguide sections to eliminate the sharp edges. The ends 31 of these broadened ridges are likewise rounded radially to avoid discontinuities. FIG. 6 depicts common wall 15 of FIG. 5 viewed from one end to illustrate the rounded ridges extending outward both top and bottom. The ridge width, it kept small compared to a half wavelength, avoids interference with odd mode operation.

FIG. 7 depicts a waveguide hybrid coupler similar to that of FIG. 1 with waist blocks for narrowing the double sectionwidth in the region of aperture 33. Waist block 34 is matched by a similar facing block on the opposite wall of the double section. The double section width of a hybrid coupling is normally 1.27 i10% of the center frequency wavelength. Since this factor is constant, it frequently calls for nonstandard size. Thus narrowed waist sections and transformer sections are often used.

While the invention has been described with respect to a few specific embodiments, numerous variations are contemplated Without departing from the invention. For example, the hybrid junction may -be bent or terminated in a single casting with a stepped impedance transformer or any other conventional form of waveguide termination. The whole or any part of the junction may be tapered. The junction can also be connected with multiple sections forming multiple junctions. Thus it is intended to cover the invention broadly within the spirit and scope of the appended claims.

I claim:

ll. A directional hybrid junction in rectangular waveguide operative over a spectrum of frequencies centered at a predetermined midband frequency comprising:

(a) two parallel sections of rectangular waveguide joined by and symmetrical about a common narrow wall;

(b) a single undivided slot-ridge aperture located in said narrow wall characterized by a height less than the height of said narrow wall and at least as high at every point along its length as at its ends said length 'being in the range of .85 to .95)\ where A is the free space wavelength of said midband frequency said aperture further characterized by a height to length ratio providing two directivity peaks in the response curve of the hybrid junction said slot-ridge aperture having top and bottom ridges of the same thickness as said common narrow wall and extending the full length of the aperture.

2. A hybrid junction according to claim 1 wherein said junction is substantially rectangular.

3. A hybrid junction according to claim 1 wherein said r aperture is substantially rectangular.

References Cited UNITED STATES PATENTS Miller 33311 XR Bowen et a1 33311 XR Riblet 333-11 Hatcher 33311 XR Tomiyasu 333-98 XR Marcatili 33311 XR Salzberg 333-11 XR 6 FOREIGN PATENTS 913,803 12/1962 Great Britain.

OTHER REFERENCES 5 Waveguide Handbook, Marcuvitz, McGraw-Hill, New

York (*1951), pp. 379-380.

The Short-Slot Hybrid Junction," Riblet, Proceedings of the IRE, February 1952, vol. 40, No. 2, pp. 180-184.

10 HERMAN KARL SAALBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner 

